STORY ARCHIVE

Planet Of The Rings

Saturn. Girdled by its mighty rings, this is the most instantly recognisable yet least understood of the planets. Now, after a 7 year journey through the solar system, the Cassini spacecraft has finally arrived for what should prove to be an unforgettable four year orbital mission:

“For everybody just peering over our shoulders looking at the images, they’re going to see exploration at its finest,” says Imaging team leader Dr Caroline Porco.

Cassini is the most sophisticated planetary explorer ever built, bristling with scientific apparatus and some of the sharpest cameras yet flown. It also carries a second spacecraft, the Huygens probe. Huygens is hitching a lift to Saturn’s largest moon: the enigmatic Titan. Titan is the only moon in the Solar System with a dense atmosphere which makes it of tremendous interest to those searching for life beyond the Earth.

While simply reaching Saturn was an accomplishment in itself, the fate of the entire US$3.5 billion expedition depended upon Mission Control being able to thread an unmanned spacecraft travelling at 80,000 kilometres per hour through a gap in the rings of Saturn without hitting even a speck of debris – all at a distance of over a billion kilometres. Failure would be absolute. Success, an historic moment in the annals of space exploration.

TRANSCRIPT

NarrationImagine taking the holiday from hell. To a world where the wind can scream at one and half times the speed of sound. Where the temperature is 180 below and cyclones are as big as the earth.

Now, imagine that beneath this swirling maelstrom there is no solid ground to stand on, only an infernal descent into molten, liquid gas.

Such a strange world not only exists, but we're about to see all the stunning postcards.

For the Earth has sent a spacecraft to the one true Lord of the Rings, deep into the realm of the planet Saturn...

NarrationOn the evening of June 30th, 2004 - after a seven year voyage through the solar system - the Cassini spacecraft hurtled towards a fateful rendezvous.

Todd Barber:The hopes and dreams of thousands of scientists and engineers are resting on the next few moments.

NarrationAfter a 3 billion kilometre journey, this is Cassini's moment of truth.In a few moments it should start the main rocket motor for its burn.

In the next two nail-biting hours the spacecraft would have to run the deadly gauntlet of Saturn's mighty rings. If it failed, if it hit a piece of debris even the size of a poppy seed, the most ambitious interplanetary expedition in the history of space exploration would be in ruins...

Cassini's long journey to Saturn began in 1997, after a planning and construction phase stretching back 14 years before that. Finally, the largest and most sophisticated planetary explorer ever built was ready for take off.

Torrence Johnson: Basically going into space is a risky business. You've got to get yourself off theEarth by putting yourself on top of a lot of high explosives lighting the fuse.All you can do is sit back and chew your nails frankly.

We have lift-off.

Jonathon Lunine:"It was very eerie because you could only see this - off in the distance - this illuminated launch pad. And with the countdown there is this bright flame, but itwas very far away. And I thought I'm glad it's going off, but this is kind of visually a bust.

But then, just as it got above the tower the rocket went into a cumulus cloud and it began to light the cumulus cloud from all sorts of different angles as it went through, and that was just the most amazing thing. And at that point I had the sense of, well here we go, we're off to Saturn.

NarrationFour times further than the asteroid belt, twice as far as Jupiter, Saturn is long way away. Yet it has always commanded our attention...

Andy Ingersol:The first time I saw it in a telescope I think true of everyone I was just bowled over by the rings and the shadow of the rings on the planet. It's a beautiful object just because of the rings.

The rings are actually very interesting because they block the light for half a Saturn year the northern hemisphere is blocked out by the rings and the other half of the year it's the southern hemisphere and we don't quite know if that has any effect on the meteorology.

Carolyn Porco:Case in point, Saturn's equatorial jet blows around the planet at 1200 mph. It's the windiest place in the solar system and we don't understand how that comes about.

Narration:A year on Saturn lasts for nearly 30 earth years, yet the planet spins so fast a day is less than 11 hours long. But Saturn gets much weirder than this...

Jonathon Lunine: Saturn is the second largest planet in the solar system. It's the gas giant planet like Jupiter which means that it's made of the lightest elements in the cosmos, hydrogen and helium and so it's largely gas.

Torrence Johnson: People are always asking me, well what would you find if you landed on Saturn? Well you can't land on Saturn. Saturn doesn't have a surface. All it's got is clouds and gas. The gas gets compressed more and more as you go in.

So the interiors of these things are really much more like a star. It just doesn't happen to be doing thermo nuclear burning.

Narration:Much of what we know about the Saturnian system was gained over the few short days of the Voyager flypasts, over 20 years ago.

First Voyager 1 then Voyager 2 swept in past the ringed planet sending back the first close images before heading off into deep space. Here in all its majesty was Saturn and its rings, far more elaborate than expected.

Here also were the first glimpses of some of Saturn's retinue of 31 known moons: moons like Tethys, Mimas and the mysterious, tantalysing Titan.

Torrence Johnson was then part of the Voyager imaging team...

Torrence Johnson: After the Voyager missions which were basically reconnaissance flybys we knew we were going to want to go back to some of these places.""Now having gotten that first look and our appetites whetted it's feeling really great 20 years after the Voyager mission to get back to Saturn and be able to really investigate it in detail.

Narration:Cassini may be light years ahead of Voyager technologically, but it didn't have the velocity necessary to travel directly to Saturn.

From launch, Cassini headed in to Venus to pick up the first in a series of gravitational slingshots. After being flung around Venus twice and the Earth once it had the gravity boosts needed to reach the outer solar system.

By the year 2000 it found itself sweeping past Jupiter. It was a perfect chance for a performance check.

Caroline Porco knew that Cassini's cameras were technically better than anything that had been out here before...

Carolyn Porco:Knowing all that didn't prepare us for what it was going to be like to see that firstimage of Jupiter come down, I'll never forget that, when it just was mind-blowingly detailed and we're so far away I literally, I mean literally tears came to my eyes, I couldn't believe what I was seeing.

Narration:Jupiter was the halfway mark. From here it was only another 4 years and further billion kilometres to go.

In many ways, the countdown to Saturn started 4 centuries ago. Astronomer Jean Dominique Cassini discovered four of the moons of Saturn and a gap in the rings that, like the spacecraft, now bears his name.

The other man who dreamed of exploring Saturn at the time was Dutchman, Christiaan Huygens.

Ralph Lorenz:Christiaan Huygens imagined planets as places. He actually wrote about what it would be like to be on Saturn, what the raindrops should be like on Saturn. He felt that it would have to be made of something other than water because Saturn is so far from the sun and it's so cold.

Narration:It was Huygens who first resolved the nature of the rings and who discovered the largest of Saturn's many moons, the cloud-shrouded Titan. Clues to the origin of life may lurk on its surface...

Carolyn Porco:Titan is the body in the solar system that comes closest to being like the earth than any other.

We think that Titan could have on its surface and in its atmosphere, materials or processes going on that went on in the early earth prior to the emergence of life so it's very intriguing.

Scientists suspect that below this smoggy veil is an icy, primitive world abundant with the building blocks of life. A frozen chemical laboratory where methane falls as rain, and oceans are filled with hydrocarbons.

Ralph Lorenz:Titan right now is just a sort of abstract concept, it's just a dot in the telescope, but that's about to change it's going to be transformed into a place you can imagine being, as a world in its own right.

NarrationRevealing the secrets of Titan has become a prime focus of the international Cassini-Huygens mission to Saturn.

Riding piggyback on NASA's Cassini is a second spacecraft - the Huygens probe - built by the European Space Agency.

Huygens' task is to make a high-risk descent to the hidden surface of Titan.

In January next year it will parachute down through the nitrogen rich atmosphere. But even if the probe survives touchdown, or possibly splashdown - the mission will last less than 3 hours before the batteries freeze and the Cassini mother-ship sails out of range - a long wait for such a short visit...

Torrence Johnson: You have to be patient with these things. Typically 20 years between the time you start thinking about a mission and when you've actually got something at the planet taking data.

Narration:After two thousand four hundred and thirty one days travel, Cassini finally approached the outskirts of the Saturnian system on the 11th of June. Crossing the city limits, it encountered the small moon Phoebe.

Astronomers knew Phoebe didn't belong here: it was lumpy, only 220 kilometres across and orbited in the wrong direction. Time for Cassini to swing into action.

Carolyn Porco:We're pretty certain now it's an object that came from the distant outer solar system, is one of these objects that has been called for decades a building block of the planets in the outer solar system. So our encounter with Phoebe was finally putting a face to the name. It looked like you could reach out and touch it.

Narration:Cassini was now travelling over a million kilometres a day as Saturn's immense gravity began to draw it in.

Three days out and Cassini recorded the sounds as it broke through the leading edge of Saturn's invisible magnetosphere.

With arrival in less than 12 hours, the imaging team meet for a final planning session before Cassini's 4 year tour of Saturn begins.

If all goes well tonight, this team will be kept very busy.

Carolyn Porco:Our job is to decide what pictures need to be taken of what targets and when and how to plan the sequences" "We could see half a million pictures easily.

Narration:The encounter with Phoebe was merely the first in a long series of moon flybys that have been planned down to the last second for years...

Carolyn Porco:Studying those moons around Saturn is going to be like studying the planets around our Sun. We should be able to understand the geological processes that have gone on, on these bodies.

Even on approach to Saturn we have taken images of Titan. We are starting to make out a pattern around the equator of Titan.

Narration:To see any more, Cassini must do what no spacecraft has ever done before: slip into orbit around Saturn.

Carolyn Porco:This is it. Being at this juncture about to undertake a very dangerous passage which may not in fact turn out reminds me of what Tom Hanks, playing the role of Jim Lovell in the movie Apollo 13 had to say to his fellow astronauts as they were making their final approach to Earth not knowing if they were going to get through the atmosphere, he said "Gentlemen and ladies, it's been a pleasure flying with you." Cheers!

Narration:The fate of Cassini's seven year journey through the solar system will be decided in the next few hours of the orbital insertion manoeuvre.

Torrence Johnson: We're coming in from below the rings seen from the spacecraft. And we have to actually go through the plane of the rings on our way up to where we want to be at closest approach to Saturn.

Narration:This is navigation at its most extreme: Saturn's rings might be beautiful but they are deadly for a spacecraft. Each band is composed of trillions of lumps of ice; some as small as snowflakes, some as big as houses. Running into even a pea-sized piece at Cassini's speed would be catastrophic.

Narration:Cassini heads in at over 70,000 kilometres per hour, aiming for the gap between the outer F and G rings clear of orbital debris - or so they hope....

Torrence Johnson: As a precaution the engineers are going to turn the spacecraft so that our large main antennae is actually pointed in the direction of the spacecraft motion as we're crossing the ring plane.

The spacecraft team has done these manoeuvres before, they have practised them, we have simulated them in computers. But you're always sitting there saying 'boy, I hope it works'.

Narration:Even if it hasn't worked, it will take 84 minutes for mission control to find out. At this stage Cassini is on its own.

Tod Barber Mission Control: Doug Johnson just received a signal. We've survived the crossing of the F & G rings.

Mission Control:Believe me there's more to come.

Narration:We've just heard that it has survived the first ring plane crossing. It is now travelling at over 80,000 kilometres per hour. In a few moments it will fire its main engines. At this stage the only thing mission control can do to help is remember to breathe.

Todd Barber:We need to let Saturn's gravity capture us into orbit.We have to slow down the spacecraft otherwise we fly by never to see the ringed planet again. We have to slam on the brakes tonight with a 96 minute burn.And it has to go perfectly tonight.

Narration: The success of the retro burn is now in the hands of Cassini's onboard computer...

Mission Control: Reports a Doppler shift.

Tod Barber in Mission Control: One minute down. 96 to go.

Narration: Minute by nail-biting minute Mission Control follows the progress of the burn, cheering as each essential milestone is reached.

Mission Control: We've got this long burn to come and the descending ring plane crossing. We'll hold our breath for that as well.

"It's basically sort of like standing on the bridge of the Enterprise and getting our probes out and telling us about the far reaches of the solar system."

Narration: These are the images Cassini's cameras snapped as the spacecraft raced away from the rings.

Carolyn Porco: I am surprised at how surprised I am. These images are shocking to me.

Narration: Here in unprecedented detail are the strange and precise patterns rippling through the rings. Density waves, crisp, scalloped edges and thin braided bands: all manifestations of the dynamic gravitational forces at work in the rings.

More secrets will be revealed on the 26th October when Cassini will make its first close fly past of Titan for a valuable reconnaissance prior to releasing the Huygens probe on Christmas Day.

While Huygens trip is brief, Cassini itself will orbit Saturn some 74 times, with 45 flybys of Titan alone. The spacecraft should remain operating until at least 2008. The data it returns should more than make up for all the effort taken to get it there...

Torrence Johnson: Missions to the outer solar system are just not simple. You want to make sure you're taking along all the right stuff with you and you're going to do a really significant mission once you're there.

Jonathon Lunine:There will never be another time when this is the first moment that the solar system is being explored.

Caroline Porco:For everybody just peering over our shoulders looking at the images, they're going to see exploration at its finest.

Narration:Cassini's grand tour of Saturn has just begun. Soon the results will be streaming in, and 2004 will be remembered as the year Planet Earth went Ringside...